runtime, runtime/cgo: track memory allocated by non-Go code

Otherwise a poorly timed GC can collect the memory before it
is returned to the Go program.

R=golang-dev, dave, dvyukov, minux.ma
CC=golang-dev
https://golang.org/cl/6819119

src/pkg/runtime/cgo/callbacks.c

@@ -33,7 +33,13 @@
 static void
 _cgo_allocate_internal(uintptr len, byte *ret)
 {
+	CgoMal *c;
+
 	ret = runtime·mal(len);
+	c = runtime·mal(sizeof(*c));
+	c->next = m->cgomal;
+	c->alloc = ret;
+	m->cgomal = c;
 	FLUSH(&ret);
 }
 

src/pkg/runtime/cgocall.c

@@ -135,6 +135,8 @@ runtime·cgocall(void (*fn)(void*), void *arg)
 		g->defer = &d;
 	}
 
+	m->ncgo++;
+
 	/*
 	 * Announce we are entering a system call
 	 * so that the scheduler knows to create another
@@ -150,6 +152,14 @@ runtime·cgocall(void (*fn)(void*), void *arg)
 	runtime·asmcgocall(fn, arg);
 	runtime·exitsyscall();
 
+	m->ncgo--;
+	if(m->ncgo == 0) {
+		// We are going back to Go and are not in a recursive
+		// call.  Let the GC collect any memory allocated via
+		// _cgo_allocate that is no longer referenced.
+		m->cgomal = nil;
+	}
+
 	if(d.nofree) {
 		if(g->defer != &d || d.fn != (byte*)unlockm)
 			runtime·throw("runtime: bad defer entry in cgocallback");

src/pkg/runtime/runtime.h

@@ -81,6 +81,7 @@ typedef struct	GCStats		GCStats;
 typedef struct	LFNode		LFNode;
 typedef struct	ParFor		ParFor;
 typedef struct	ParForThread	ParForThread;
+typedef struct	CgoMal		CgoMal;
 
 /*
  * Per-CPU declaration.
@@ -249,7 +250,9 @@ struct	M
 	int32	profilehz;
 	int32	helpgc;
 	uint32	fastrand;
-	uint64	ncgocall;
+	uint64	ncgocall;	// number of cgo calls in total
+	int32	ncgo;		// number of cgo calls currently in progress
+	CgoMal*	cgomal;
 	Note	havenextg;
 	G*	nextg;
 	M*	alllink;	// on allm
@@ -414,6 +417,14 @@ struct ParFor
 	uint64 nsleep;
 };
 
+// Track memory allocated by code not written in Go during a cgo call,
+// so that the garbage collector can see them.
+struct CgoMal
+{
+	CgoMal	*next;
+	byte	*alloc;
+};
+
 /*
  * defined macros
  *    you need super-gopher-guru privilege